In circuitry that receives an AC signal (“AC-coupled circuitry”), a DC component of a complex wave (i.e., one containing both AC and DC components) is a current component that has an unchanging polarity. That is, the DC component is the mean (average) value at which the AC component(s) alternate, pulsate, or fluctuate. An AC waveform with a zero DC component (i.e., an AC waveform having an average value of zero) is known as a DC-balanced waveform. In waveforms that are not DC-balanced, a DC offset can be introduced to counter the DC component and balance the waveform, making accurate DC component determination particularly important. DC-balanced waveforms can be used in AC-coupled circuitry (e.g., optical or electrical communication and/or storage circuits) to avoid voltage imbalance problems between connected systems and components.
In certain AC-coupled circuitry (e.g., optical detectors, optical transmitters, etc.), an off-state DC level is introduced by the AC-coupled circuitry itself. For example, in optical detectors, an unwanted DC level is introduced by dark current. Dark current is the electrical current that flows through the detector when the detector is not exposed to light or a light signal. Introduction of the unwanted DC level increases the DC component of the AC signal. An accurate determination of the DC level can be particularly important in seeking to avoid voltage imbalance problems. Accurate DC level determination can also be particularly important in optical or electrical communication circuitry (e.g., an optical transmitter) having a strict extinction ratio (ER) requirement.
An ER is a ratio of the maximum power of an AC signal to the minimum power of the AC signal. In many AC-coupled systems, the minimum zero-level power P0 is near the off-state power (e.g., POFF) of the AC-coupled circuitry in the absence of the AC signal. The extinction ratio is generally expressed as a fraction in dB, or as a percentage, and can be calculated according to Equation [1] below:ER=(P1−POFF)/(P0−POFF)  [1]where POFF is the off-state power, P1 is the maximum power of the AC signal, and P0 is the minimum power of the AC signal. The minimum or zero-level power P0 is oftentimes very close to the off-state power POFF (e.g., the power leaked by the AC-coupled circuitry). Thus, utilizing an inaccurate POFF value in Equation [1] can result in the denominator having a calculated value significantly different from the actual value, resulting in inaccurate ER values.
Conventional methods of determining a DC level or DC component (e.g., an off-state power POFF) in an AC or AC-coupled signal may include the use of a DC probe, a signal tap, or additional circuitry to mirror or directly measure the DC component. However, such methods may inaccurately determine the off-state power (e.g., POFF) as having the same value as that of the zero-level power (P0), resulting in an inaccurate ER value (e.g., an ER value calculated using Equation [1]). That is, such methods and circuitry do not always accurately detect the DC level of the AC-coupled signal that is introduced by the AC-coupled circuit itself. Such methods can cause detrimental effects to the signal quality of the received AC signal, especially in high-frequency or high data-rate systems, resulting in inaccurate DC level determinations. Additionally, such techniques can add unwanted complexity to a product including the AC-coupled circuitry. Thus, although DC-coupled designs can greatly simplify the measurement of the DC component or DC offset, such designs add greater complexity in the AC performance of the system.
For example, utilizing a DC probe to determine a DC level may result in an inaccurate DC voltage measurement due to the internal circuitry of the DC probe. The forward voltage drop (e.g., about 0.70V) of an internal diode in the DC probe may limit accurate measurements of voltages less than or about equal to the forward voltage drop of the diode. Additionally, DC component measurements utilizing a signal tap or additional circuitry (e.g., a DC mirror circuit) may decrease the accuracy of the measurement due to the introduction of additional loads (e.g., impedance) or noise on the AC-coupled circuitry.
This “Background” section is provided for background information only. The statements in this “Background” are not an admission that the subject matter disclosed in this “Background” section constitutes prior art to the present disclosure, and no part of this “Background” section may be used as an admission that any part of this application, including this “Background” section, constitutes prior art to the present disclosure.